The electrolytes used in electrochemical devices such as electrical double-layer capacitors and nonaqueous electrolyte secondary cells have until now been electrolytes obtained by dissolving a solid electrolyte salt in a nonaqueous solvent or solid electrolytes obtained by rendering such a solution into a solid with a polymer.
Liquid electrolytes lack long-term reliability because nonaqueous electrolyte solutions readily volatilize and are flammable, in addition to which liquid leakage may occur. Solid electrolytes resolve such drawbacks of liquid electrolytes and enable the production process to be simplified. Moreover, they offer the added advantage of making it possible to achieve thinner, smaller, and lighter weight devices. Yet, electrolyte salts lack sufficient solubility in the nonaqueous solvents which are used together with these solid electrolytes, limiting the amount of such salts that may be added. Consequently, the resulting electrolytes have a low ionic conductivity, and electrical double-layer capacitors and nonaqueous electrolyte secondary cells made therewith have a low capacitance or capacity. Moreover, because the electrolyte salt has a low solubility, it readily settles out of solution at low temperatures, which adversely affects the low-temperature properties of devices such as electrical double-layer capacitors.
In searching for a solution to these problems, various electrolytes that use ionic liquids as the electrolyte salt have been studied. Ionic liquids, which are liquid at ambient temperatures, have many desirable characteristics, including (1) absence of a vapor pressure, (2) high heat resistance and broad liquid temperature range, (3) non-flammability, (4) chemical stability, (5) high ionic conductivity, (6) high decomposition voltage, and (7) handleability in air. Such qualities have led recently to a broad and growing recognition of the usefulness of ionic liquids.
Efforts are underway to exploit these attributes by using ionic liquids in various applications, such as solvents for organic synthesis, including solvents for catalytic reactions; highly stable, non-volatile recyclable “green” solvents targeted at material separation and recovery; and novel electrolytes for use in electrochemical devices.
Of these applications, particularly rapid progress is being made in research on the use of ionic liquids as electrolytes for electrical double-layer capacitors or nonaqueous electrolyte secondary cells.
However, such electrolytes, being liquids, may give rise to problems such as liquid leakage that are associated with liquid electrolytes. Recently, to satisfy the desire for higher safety, investigations have also been conducted on solid electrolytes obtained by converting ionic liquids into solids.
For example, JP-A 2002-3478 (Patent Reference 1) discloses an ionic gel obtained by dissolving a polymer in an ionic liquid composed of an imidazolium salt, JP-A 7-118480 (Patent Reference 2) discloses a solid polymer electrolyte obtained by dissolving a polymer having an alkyl quaternary ammonium salt structure in an ionic liquid composed of a nitrogen heterocycle-type quaternary ammonium salt, and JP-A 10-83821 (Patent Reference 3) discloses a solid polymer electrolyte obtained by reacting an imidazolium derivative with monomers to prepare a fused-salt monomer, then polymerizing the fused-salt monomer.
However, when efforts are made to dissolve polymers or monomer compounds in ionic liquids by prior-art methods, dissolution is known to be difficult or impossible owing to the very poor compatibility of the two components. The solid polymer electrolytes disclosed in above Patent References 1 to 3 are also affected by this problem.
In light of these circumstances, one object of the present invention is to provide polymer electrolyte-forming compositions which can be rendered into a polymer without compromising the excellent properties of the ionic liquid, which have an excellent safety and electrical conductivity, and which form electrolytes having a broad potential window. Additional objects of the invention are to provide polymer electrolytes obtained from such compositions, and also electrical double-layer capacitors and nonaqueous electrolyte secondary cells in which such polymer electrolytes are used.